During the manufacture of semiconductor elements and liquid crystal display elements, a projection apparatus is used to project the image of a detailed pattern formed on a photomask or a reticle (hereafter referred to as a reticle) onto a photosensitive substrate (hereafter referred to as a wafer) such as a semiconductor wafer or a glass plate coated with a photosensitizer such as a photoresist. The reticle pattern is projected using an exposure apparatus such as a step and repeat system exposure apparatus, where the reticle and the wafer are positioned relatively with a high degree of precision (alignment), and the reticle pattern is then exposed so as to overlap the patterns already formed on the wafer. The requirements for this alignment process are becoming more and more stringent as patterns are miniaturized, and many schemes have been devised for the alignment process.
The most general method of reticle alignment utilizes an exposure light. Examples of this type of alignment system include the VRA (Visual Reticle Alignment) system wherein the exposure light is irradiated onto an alignment mark drawn on the reticle, and the mark position is measured by processing of the image data of the alignment mark captured by a CCD camera or the like.
Examples of wafer alignment systems include LSA (Laser Step Alignment) systems such as that disclosed in Japanese Unexamined Patent Application, First Publication No. Sho 60-130742 wherein laser light is irradiated onto a dotted line shaped alignment mark on the wafer, and the light diffracted or scattered by the mark is used for detecting the mark position, FIA (Field Image Alignment) systems such as that disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 4-65603 wherein light of a broad wavelength band from a light source such as a halogen lamp is used for the illumination, and the mark position is then measured by image processing of the image data of the alignment mark captured by a CCD camera or the like, and LIA (Laser Interferometric Alignment) systems such as those disclosed in Japanese Unexamined Patent Application, First Publication No. Sho 61-208220 and Japanese Unexamined Patent Application, First Publication No. Sho 61-215905 wherein two coherent beams which are inclined relative to the pitch direction are irradiated onto a diffraction grid shaped alignment mark on the wafer, the two diffracted light beams undergo interference, and the phase of that interference is used for measuring the position of the alignment mark.
In these types of optical alignments, first the alignment mark on the reticle is detected, processed, and the position coordinates are measured. Next, the alignment mark on the wafer is detected, processed, and the position coordinates are measured to determine the position of the shot for registration. Based on these results, a wafer stage is used to move the wafer so that the reticle pattern image is superimposed on the shot position, and the reticle pattern image is then exposed.
However, the conventional position measuring devices, position measurement methods and exposure apparatus described above suffer from the problems described below.
Exposure apparatus are typically provided with the aforementioned plurality of different types of sensors as part of the alignment system, but in all such systems, the influence of the marks and optical aberrations mean that the image obtained is asymmetric, leading to positional deviation errors, and a deterioration in the alignment precision. An example of a method for reducing these positional deviation errors comprises conducting measurements by using a shorter period (high frequency) component of the image. In the LSA system and the FIA system, raising the NA value (numerical aperture) and shortening the wavelength corresponds with this measurement method. In the LIA system, detecting the phase of higher order diffracted light corresponds with this measurement method.
However, in these measurement methods, there are associated manufacturing limits. Furthermore, this method also suffers from a deterioration in the S/N ratio of the high frequency component, which incorporates surface roughness on the wafer as noise.
In terms of the aforementioned manufacturing limits, in the LSA system and the FIA system, positional deviation errors can be reduced if the NA of the detection optical system such as a lens is increased, but the NA cannot be increased indefinitely. The reason being that the greater the value of the NA, the larger the entire alignment device becomes. However, alignment devices using a LSA system or a FIA system are typically positioned inside the exposure apparatus in a confined space below the projection lens (in the vicinity of the wafer stage), and so the alignment device cannot be increased in size indefinitely.
Furthermore, in the LSA system and the FIA system it is already proven that the positional deviation error can be reduced by shortening the wavelength of the detecting light. Shortening the wavelength can be achieved by, for example, using a light source which emits a shorter wavelength light, but there is a restriction in that light of a wavelength which will expose the wafer (exposure light) cannot be used. Furthermore, from a technical viewpoint, it is not currently possible to use all light sources which emit a short wavelength, and there are limits on the numbers of short wavelength light sources which can be practically applied as the alignment light source.
Moreover in the LIA system, it is already proven that positional deviation errors can be reduced by detecting the phase of higher order diffracted light. This detection of higher order diffracted light is closely related to the NA of the detection optical system. As the order of diffracted light increases, the emission (reflection) off the alignment mark becomes increasingly broad. In order to detect this very broad band of high order diffracted light, a detection optical system with a large NA must be used. However as described above, the NA value cannot be increased indefinitely. Consequently, the order of diffracted light which can be detected is also necessarily limited.
Furthermore, the problems described above also arise in the case of superposition measuring devices for measuring the superposition error of exposed patterns across a plurality of layers on a wafer surface.